The number one concern is about "the present satellite and missile capacity of North Korea to detonate nuclear bombs over the US knocking out the entire electronic infrastructure through an electromagnetic impulse (EMP) attack liquidating 90% of the American population within a year..."

Equipment that is running at the time of an EMP is more vulnerable. Even a low-energy pulse has access to the power source, and all parts of the system are illuminated by the pulse. For example, a high-current arcing path may be created across the power supply, burning out some device along that path. Such effects are very hard to predict, and equipment needs high-level laboratory testing for vulnerabilities if it may be subjected to pulses.[38]"

A high-altitude electromagnetic pulse (HEMP) is a NEMP warhead designed to be detonated far above the Earth's surface. The explosion releases a blast of gamma rays into the mid-stratosphere, which ionizes and the resultant energetic free electrons interact with the Earth's magnetic field to produce a much stronger EMP than is normally produced in the denser air at lower altitudes.